Resolving Horse Choke with an Antispasmodic Drug (AAEP 2011)

Esophageal obstruction, or "choke," is a common occurrence in horses that veterinarians approach with a number of treatments from passing a nasogastric tube to sedation and other drug approaches. At the 2011 American Association of Equine Practitioners convention, held Nov. 18-22 in San Antonio, Texas, Joe Bertone, DVM, MS, Dipl. ACVIM, presented information about the use of the antispasmodic N-butylscopolammonium bromide (Buscopan)—a drug generally used in colic cases—as an effective treatment to relieve simple choke.

Bertone explained that Buscopan is known for its ability to relax smooth muscle and is useful in managing and diagnosing abdominal pain (colic). Veterinarians often use the drug to reduce the risk of rectal tear during palpation and when performing a variety of obstetrical procedures including twin reduction and dystocia (difficult birth) management. Anecdotally, Buscopan has been reported effective in relieving meconium (the first manure a foal produces) and ileal impactions, he said.

Buscopan has been accepted as effective treatment for simple choke in other countries for 40 years, Bertone noted, but interestingly, the drug was not designed to have an effect on the proximal (upper) skeletal muscle esophagus from the back of the mouth, along the neck, and to the heart ("This is the area where most chokes occur," he added). Rather, he said, Buscopan is designed to relax the smooth muscle of the distal (lower) third of the esophagus (from the heart to the stomach).

"The proximal area is where choke (typically) occurs, yet the drug exerts relaxation in the last portion of the esophagus," he said. "That is puzzling."

To try to get to the bottom of the puzzle, Bertone and colleagues performed two separate studies. In the first study the team passed a polyethylene tube with an attached latex balloon into the distal third (smooth muscle) portion of the esophagus in each of six horses. The inflated balloon simulated a blockage of the esophagus. Then the team treated the horses with intravenous administration of the label dose of Buscopan. They found that within 30 seconds all esophageal swallowing ceased in the distal third smooth muscle section. Bertone commented that esophageal contractions persisted in the control horses that received no medication, although within five minutes there was a slight degree of esophageal muscular adaptation to the presence of the obstruction.

In the second study Bertone and colleagues showed that Buscopan did not alter swallowing nor pressures in the proximal portions of the esophagus lined with skeletal muscle. He theorized that the positive clinical effects seen might be explained by complete relaxation of the distal esophagus by Buscopan allowing the proximal esophagus to pass the obstruction through.

Bertone postulated another theory as well. Food obstructions, he noted, should be accompanied by aggressive esophageal swallowing (this is also what appears to be seen clinically). Bertone explained the dynamics: "Logically, there should be coordination of contraction in the proximal and distal esophagus. When a feed bolus obstructs the esophagus at the thoracic inlet (where the esophagus begins to enter the chest at the end of the neck), it is possible that the distal esophagus exhibits aggressive swallowing contractions. At some point the skeletal muscle contractions become less effective in moving a feed bolus forward. Relaxing the distal esophagus may 'reset' the contractile forces of the proximal esophagus to be more effective."

Another medication often used to treat choke is oxytocin; however, recent research has noted that the drug has little effect on the esophagus. Sedation with xylazine also stops all esophageal contractions but paralyzes the entire esophagus, necessitating passage of a nasogastric tube to “push” an obstruction to the stomach, he said.

Bertone and colleagues postulated the distal esophageal relaxation as the mechanism for the clinical effectiveness of Buscopan for choke.

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.